Semiconductor wafer processing operations involve the performance of various types of processing steps or sequences upon a semiconductor wafer upon which a number of die (e.g., a large or very large number of die) reside. The geometrical dimensions, linewidths, or feature sizes of devices, circuits, or structures on each die are typically very small, for example, micron, submicron, or nanometer scale. Any given die includes a large number of integrated circuits or circuit structures that are fabricated, processed, and/or patterned on a layer-by-layer basis, for instance, by way of processing steps performed upon wafers sitting on planar wafer surfaces, such that the dies carried by the wafer are collectively subjected to the processing steps.
A wide variety of semiconductor device processing operations involve a number of handling systems that perform wafer or film frame handling operations which involve securely and selectively carrying (e.g., transporting, moving, displacing, or conveying) wafers or wafers mounted on film frames (hereafter referred to as “film frame” for brevity) from one position, location, or destination to another, and/or maintaining wafers or film frames in particular positions during wafer or film frame processing operations. For instance, prior to the initiation of an optical inspection process, a handling system must retrieve a wafer or a film frame from a wafer or film frame source such as a wafer cassette, and transfer the wafer or film frame to the wafer table. The wafer table must establish secure retention of the wafer or, film frame to its surface prior to the initiation of the inspection process, and must release the wafer or film frame from its surface after the inspection process is complete. Once the inspection process is complete, a handling system must retrieve the wafer or film frame from the wafer table, and transfer the wafer or film frame to a next destination, such as a wafer or film frame cassette or another processing system.
Various types of wafer handling systems and film frame handling systems are known in the art. Such handling systems can include one or more mechanical or robotic arms configured for performing wafer handling operations which involve the transfer of wafers to and the retrieval of wafers from a wafer table; or performing film frame handling operations which involve the transfer of film frames to and the retrieval of film frames from a wafer table. Each robotic arm includes an associated end effector which is configured for retrieving, picking up, holding, transferring, and releasing a wafer or a film frame by way of the application and cessation of vacuum force relative to portions of the wafer or film frame, in a manner understood by one of ordinary skill in the relevant art.
A wafer table itself can be viewed or defined as a type of handling system, which must reliably, securely, and selectively position and hold a wafer or film frame on a wafer table surface while displacing the wafer or film frame relative to elements of a processing system, such as one or more light sources and one or more image capture devices corresponding to an optical inspection system. The structure of a wafer table can significantly impact whether an inspection system can achieve a high average inspection throughput, as further detailed below. Furthermore, the structure of a wafer table, in association with the physical characteristics wafers and the physical characteristics of film frames, greatly impacts the likelihood that an optical inspection process can reliably generate accurate inspection results.
With respect to the generation of accurate inspection results, during an optical inspection process, a wafer or a film frame must be securely retained upon the wafer table. Additionally, the wafer table must dispose and maintain the upper or top surface of the wafer or film frame in a common inspection plane, such that the surface areas of all wafer die, or as many wafer die as possible, collectively reside in this common plane, with minimum or negligible deviation therefrom. More particularly, the proper or accurate optical inspection of die at very high magnification requires a wafer table to be very flat, preferably with a planarity having a margin of error of less than ⅓ of the depth of focus of the image capture device. If the depth of focus of an image capture device is, for instance, 20 μm, a corresponding wafer table planarity error cannot exceed 6 μm.
For handling die of very small size (eg. 0.5×0.5 mm or smaller) and/or thickness (50 μm or less—e.g., carried by a very thin and/or flexible wafer or substrate), this planarity requirement becomes even more critical. For wafers that are very thin, it is important for the wafer table to be ultra-planar, otherwise it is easy for one or more die on the wafer or film frame to become positioned out of the depth of focus. One of ordinary skill in the art will recognize that the smaller the die, the higher the magnification required, and hence the narrower the band of depth of focus in which the inspection plane must lie.
With such planarity outlined aforesaid, a wafer placed on the wafer table will lie flatly on the wafer table surface, the wafer squeezing out substantially all the air beneath it. The difference in atmospheric pressure between the top and bottom surface of the wafer when the wafer is disposed upon the wafer table results in a large force applied against the top surface of the wafer due to atmospheric pressure, holding the wafer down strongly or reasonably strongly upon the wafer table. As pressure is a function of surface area, the larger the size of the wafer, the greater the force applied downwards on the wafer. This is commonly referred to as the “inherent suction force” or “natural suction force” on the wafer. The flatter the wafer table surface, the greater the natural suction force, up to the limit defined by the finite surface of the wafer. However, the strength of such suction force depends on how flat the wafer table surface is. Some wafer tables are not that flat and may have other grooves or holes on its surface resulting in reduced suction force. As the wafer table will be repeatedly accelerated over short distances during inspection of each die, and a high vacuum force is often applied through the wafer table to the wafer table surface to the underside of the wafer to ensure that the wafer remains as planar as possible and does not move during inspection; this is notwithstanding the presence of such natural suction force.
Various types of wafer table structures have been developed in attempts to securely hold wafers or film frames during wafer or film frame inspection operations, and reliably maintain a maximum number of die in a common plane during inspection operations. However not one design exists that will allow the wafer handling system to handle both wafers and sawn wafers mounted on film frames without one or more of the problems described below. A brief description will be made of each type of existing design and their associated problems.
Several types of wafer chucks have been or are currently in use. In the past, wafers were smaller (e.g., 4, 6, or 8 inches) and significantly thicker (particularly in relation to their overall surface areas, e.g., on a wafer thickness normalized to wafer surface area basis), and each die size was larger. Present-day wafer sizes are typically 12 or 16 inches, yet the thickness of these processed wafers have been decreasing in relation to their increasing size (for instance, thicknesses of 0.70-1.0 mm for 12-inch wafers prior to thinning/backgrinding/backlapping, and 50-150 μm following thinning/backlapping are common), and die sizes (e.g., 0.5-1.0 mm square), respectively. Standard wafer sizes can be expected to further increase over time. Additionally, thinner and thinner wafers can be expected to be processed each year in response to the increasing demands and requirements of electronics and mobile phone manufacturers for thinner die/thinner components to fit into slim-built electronic devices (e.g., flat screen televisions, mobile phones, notebook computers, tablet computers, etc.). As will be explained, these factors contribute to the increasing deficiencies of current designs of wafer table to handle both wafers and film frames.
Historically, and even presently, many wafer chucks have been made of a metal such as steel. Such metal wafer chucks are inlaid with a network of grooves, usually circular grooves that are intersected by grooves radiating linearly from a central location. Through such grooves, vacuum force can be applied to the underside of the wafer, which interfaces with the wafer table surface, in order to facilitate secure retention of the wafer against the wafer table surface. In many wafer table designs, such grooves are arranged in concentric circles of increasing size. Depending on the size of the wafer, one or more grooves would be covered by a wafer when the wafer is disposed upon the wafer table surface. Vacuum can be activated through the grooves covered by the wafer to hold the wafer down during processing operations, such as wafer inspection operations. After inspection, the vacuum is deactivated and ejector pins are deployed to lift the wafer off of the wafer table surface, such that the wafer can be retrieved or removed by an end effector. As there are linear grooves radiating from the centre of the metal wafer table surface, once the vacuum is deactivated, the residual suction force associated with application of the vacuum force to the underside of the wafer is quickly dissipated. Thicker wafers are more amenable to application of significant force applied through the ejector pins to lift the wafer (against any residual suction force, if any) without breaking.
As indicated above, increasingly wafers manufactured today are thinner or much thinner than before (e.g., present wafer thicknesses can be as thin as 50 μm), and each die thereon is also increasingly smaller in size (e.g., 0.5 mm square) than in the past. Technological progression results in smaller die sizes and thinner die, which pose a problem for handling wafers by way of existing wafer table designs. Very often, backlapped/thinned or sawn wafers (hereafter simply “sawn wafers”) having die that are very small in size and/or which are very thin are mounted on film frames for processing. Conventional metal wafer tables are not suitable for use with film frames having sawn wafers mounted thereto for a number of reasons.
Bearing in mind that inspection of die involves very high magnification, the higher the magnification, the narrower an acceptable depth of focus band, range, variance, or tolerance will be for accurate inspection. Die that are not in the same plane are likely to be out of the depth of focus of an image capture device. As indicated above, the depth of focus of a modern image capture device for wafer inspection typically ranges from 20-70 μm or smaller, depending on the magnification. The presence of grooves on the wafer table surface presents problems particularly during the inspection of sawn wafers mounted on film frames (with small die sizes) on such systems.
The presence of grooves results in the sawn wafers with small die sizes not sitting properly or uniformly on the wafer table surface. More particularly, in regions where there are grooves (and there can be many), the film frame's film can slightly sag into the grooves, resulting in the whole wafer surface lacking collective or common planarity across all die, which is critical for optical inspection operations. This lack of planarity becomes more pronounced for small or very small die of sawn wafers. Furthermore, the presence of a groove can cause die to be displaced at an angle relative to a common die inspection plane, or cause the die to sag and sit at one or more different and lower planes. Furthermore, light shining on tilted die which have sagged into grooves will reflect light away from the image capture device, such that the capture of an image corresponding to a tilted die will not contain or convey precise details and/or features of one or more regions of interest on the die. This will adversely affect the quality of images captured during inspection, which can lead to inaccurate inspection results.
Several prior approaches have attempted to address the aforementioned problems. For instance, in one approach a metal wafer table support includes a network of grooves. A flat metal plate is placed on top of the network of grooves. The metal plate includes many small or very small vacuum holes that allow vacuum to be applied through the perforations against a wafer or sawn wafer. Depending on the size of wafer under consideration, an appropriate pattern or number of corresponding grooves will be activated. While multiple small or very small vacuum holes can increase the likelihood that die can be collectively maintained in the same inspection plane, collective die planarity problems are still not effectively or completely eliminated due to continuing technological evolution that results in smaller and smaller die sizes and decreasing die thicknesses over time Such designs also include multiple sets of ejector pin triplets corresponding to different wafer sizes, i.e., multiple distinct sets of three ejector pins corresponding to multiple standard wafer sizes that the wafer table is capable of carrying. The presence of numerous holes for ejector pins can also present, and quite possibly worsen, collective die planarity problems when inspecting die carried on film frames, for reasons analogous to those set forth above.
Some manufacturers use wafer table conversion kits, in which a metal wafer table with grooves is used for handling whole wafers, and a metal wafer table cover with many very small openings is used for film frame handling. Unfortunately, conversion kits require inspection system downtime due to the fact that conversion from one type of wafer table to another, and post-conversion wafer table calibration, is time consuming and done manually. Such downtime adversely affects average system throughput (e.g., overall or average throughput with respect to both wafer and film frame inspection operations considered in sequence or together), and hence inspection systems that require wafer table conversion kits are undesirable.
Other wafer table designs, such as described in U.S. Pat. No. 6,513,796, involve a wafer table receptacle that allows for different central wafer table inserts depending on whether wafers or film frames are being processed. For wafer inspection, the insert is typically a metal plate with annular rings having vacuum holes for activation of vacuum. For film frames, the insert is a metal plate having many fine holes for vacuum activation, which can still give rise to collective die nonplanarity as described above.
Still other wafer table designs, such as disclosed in U.S. Patent Application Publication 2007/0063453, utilize a wafer table receptacle having a plate type insert consisting of a porous material in which distinct regions are defined by annular rings made of a thin film material. Typically, such wafer table designs are complex in construct and involves a delicate and complex manufacturing process, and hence difficult, time consuming, or costly to manufacture. Moreover, such designs can utilize metal annular rings to facilitate regional vacuum force control across the wafer table surface in accordance with wafer size. Metal annular rings can require undesirably long planarization times, or damage a polishing device that is used to polish the wafer table surface when planarizing the wafer table surface. Furthermore, metal rings can give rise to nonplanarity due to differential material polishing characteristics across the wafer table surface, and therefore metal annular rings are unsuitable for modern optical inspection processes (e.g., particularly involving sawn wafers mounted on film frames).
Unfortunately, prior wafer table designs are (a) unnecessarily structurally complex; (b) difficult, expensive, or time consuming to fabricate; and/or (c) unsuitable for various types of wafer processing operations (e.g., die inspection operations, particularly when die are carried by a film frame) as a result of insufficient wafer table surface planar uniformity in view of technological evolution that continues to give rise to smaller and smaller wafer die sizes and/or progressively decreasing wafer thicknesses. A need clearly exists for a wafer table structure and an associated wafer table manufacturing technique that that will enable the wafer table to handle both wafers and sawn wafers and which overcomes one or more of the foregoing problems or drawbacks.